Not applicable.
Our invention is a synthetic venous valve and graft combination, useful to replace a diseased, injured, or otherwise nonfunctioning venous valve in the human vasculature.
Significant differences between the venous environment and the arterial environment should be considered when designing prosthetic venous valves. Much of vascular research and development to date has centered on the arterial system, as problems in this system tend to be life threatening. The most obvious difference between the two environments is the lower oxygen content of the venous blood. The lower oxygen content is a key factor in the exceedingly high propensity towards thrombus formation. Blood in the arterial system is moved rapidly and at high pressure by the heart. In contrast, blood is moved through the veins at slightly over half the arterial pressure (close to the measured systolic pressure) and more slowly. Blood flow rates in smaller veins can fall below one milliliter per second. Veins are surrounded by muscle that promotes blood flow during muscle tension/contraction. During muscular contraction of the vein, the change in vein diameter is large compared to arterial diameter change within a heartbeat. The venous system requires multiple valves to function whereas the arterial system lacks valves except in the heart. The normal function of venous valves is to direct the venous blood towards the heart by allowing only unidirectional flow. With the vein lumen contraction causing flow and the valves directing flow, a venous pressure is produced. It is important that the venous valves operate properly to prevent reverse flow or xe2x80x9crefluxxe2x80x9d. Some venous valves may be required to actuate with a pressure change of less than 1 mm Hg. This is barely a differential of 1.5% of the normal venous pressure.
When, due to disease, injury, or congenital defect, valves do not properly operate, reflux can occur, causing blood to pool in the body extremities. Existing forms of pharmaceutical and surgical treatments for such venous disorders all have limitations. In some cases replacement valves are required to maintain a patient""s ambulatory functions. Surgical techniques for valve replacement are complicated and delicate. Less invasive endovascular and percutaneous techniques are desirable and are known.
No currently available prosthetic valve fully addresses the special needs of the venous environment. An 1856 description of the design and operation of various xe2x80x9celastic valvesxe2x80x9d within tubes, which greatly predates all use of valves in mitral, aortic, arterial, or venous prostheses, is given by Peale (U.S. Pat. No. 15,192). Peale""s description fails to clearly show how a trilobal valve can be formed inside a tube. One innovative invention (U.S. Pat. No. 6,241,763) describes forming a venous valve insitu by overlapping the vein wall and attaching it in place to form two tubular regions from one. A consequent increase in tissue bulk and decrease in vein lumen requires greater pressures and /or will move less blood volume. Both consequences encourage blood pooling. Other valve designs tend to be scaled-down versions of heart valves. These designs require a support wire, stent, annular xe2x80x9cvalve ringxe2x80x9d, or valve seat at the site of the valve, itself (see for examples U.S. Pat. Nos. 5,358,518; 5,607,465; 6,315,793; 6,319,281; 6,168,614; 6,299,637). All such devices place radially outward forces on the inner vein wall, causing radial stiffening. With a large percentage change in diameter during pulsation, any foreign object exerting radial stiffness causes local irritation to the vein wall. Foreign body irritation engenders neointimal hyperplasia, sclerosis, and associated thrombus formation. This is the major reason that stents, which have proven lifesaving in arteries, are not routinely used in veins.
Heart valves are often constructed of fixed biological tissues (notably U.S. Pat. Nos. 5,509,930 and 6,126,686). In the low flow and low oxygen venous environment, fixed tissues tend to calcify. Calcified and otherwise stiffened valves do not function well at low pressure differentials. Although some design features of the heart valve shown in U.S. Pat. No. 5,509,930 are desirable, the use of bulky layers of fixed biological tissue with folds and overlap seams prevent this invention from application as a suitable venous valve. The valve stent invention in U.S. Pat. No. 5,957,949 is a unibody wire stent covered with a graft to which a valve is permanently attached. The stated preferred valve is a treated bicuspid porcine valve and, although the stent design and construction are detailed, no design is provided for a synthetic valve.
Our invention is a venous valve prosthesis designed specifically to meet the challenges of the venous environment: a synthetic valve and graft combination having a trilobal configuration in the actuating portion.
Objectives
With the foregoing in mind, it is an objective of our invention to design a streamlined synthetic venous valve and graft combination that applies no radial force on the vein wall at the valve site.
It is another objective of our invention to design a venous valve and graft combination that operates at pressure differentials approaching 1 mm Hg.
It is a further objective of our invention to provide a venous valve and graft combination having a small compressed bulk, and which can be flexibly tailored to meet the sizing, attachment, and delivery requirements of the patient.